1. FIELD OF THE INVENTION
The invention relates to a method and apparatus for effecting the perforating and the gravel packing of a production zone in a subterranean well by a single trip of a tubular tool string into the well which carries both perforating and gravel packing apparatus.
2. DESCRIPTION OF THE PRIOR ART
As oil and gas wells are drilled to constantly increasing depths, the cost of completion or the workover of a well is disproportionately increased by the number of trips of completion apparatus that must be made into the well in order to effect its completion or workover. Necessarily every encased producing well has to have the casing perforated in the production zone. It is equally necessary in the case of many wells having unconsolidated production formations to provide gravel packing in the area of the perforations to filter out sand produced with the production fluids and thus prevent entry of sand into the well bore and through the casing perforations into the production conduit. In co-pending application, Ser. No. 501,262, filed June 6, 1983, and assigned to the Assignee of this application (BSC-55-CONT), there is disclosed a combined gravel packing and perforating apparatus which may be run into the well and, in a single trip, effect the perforating of the well casing, and the gravel packing of the perforations and a screen attached to the bottom of the tubular tool string.
Necessarily a substantial amount of apparatus has to be assembled on the tool string to produce the one trip gravel packing and perforating apparatus. Hence, the run-in of such a substantial length of tools in a tubular string generally requires a slower operation than when a perforating gun or gravel packing apparatus is run in individually. Nevertheless, if the gravel packing and perforating of the well casing can be accomplished in a single trip of the lengthy tool string into the well, it is economically desirable. Unfortunately, all of the economic advantages would be lost if the perforating gun fails to fire, once it is in its proper position relative to the production formation.
This is particularly true when the perforating gun is fired by dropping a detonating bar through the tubular string and the assembled gravel packing and perforating apparatus. There are a large number of reasons why a detonating bar will arrive at the perforating with insufficient energy to discharge the impact actuated primer. For example, the well bore may have substantial deviations from the vertical which would substantially slow down the downward speed of the detonating bar. Particulate material or debris may collect in the perforating gun around the firing mechanism and absorb or cushion the impact of the detonating bar, thus resulting in insufficient impact energy to actuate the primer.
In the co-pending application, Ser. No. 593,364 filed concurrently herewith (BSC-102), and assigned to the Assignee of this application, there is disclosed and claimed a fluid pressure actuated firing mechanism wherein a firing hammer may be repeatedly actuated by reversing the fluid pressure differential acting on the hammer to return it to an elevated position with respect to the firing pin if the primer fails to discharge. It is therefore desirable to employ this type of firing mechanism in a single trip perforating and gravel packing apparatus; however, such single trip apparatus generally includes a fluid pressure actuated packer above the screen which, if fluid pressure were employed to actuate the firing mechanism for the perforating gun, the same fluid pressure would result in prematurely setting the packer, thus requiring considerable mechanical manipulation to unset the packer in order to shift the screen to a position adjacent the casing formations resulting from the firing of the perforating gun.
Accordingly, there is a need for a fail-safe type of single trip combined gravel packer and perforating gun which may be run into the well with the assurance that the gun can be eventually fired by a fluid pressure actuated mechanism permitting repeated attempts to discharge the detonatable primer to be made without disturbing the position of the perforating gun in the well casing or prematurely setting a packer.